Oxygen and nitrogen are among the most widely used chemicals in the world, the annual consumption of each gas amounting to in excess of 20 million tons in the United States. Most of this oxygen is used in the steel industry and related metals manufacturing processes. Oxygen-enriched air has also found significant uses, including treatment of waste water, non-ferrous smelting, glass production, medical applications, and other chemical oxidation processes. In addition, there is a great potential market for oxygen-enriched air in the synthetic fuels industry. Nitrogen and nitrogen-enriched air are useful primarily for inert blanketing atmospheres and for refrigeration.
More than 99% of all oxygen and nitrogen is currently produced by cryogenic fractionation, or a process involving lowering the temperature of air sufficiently (to about -215.degree. C.) to liquefy it and then using a multistage distillation process to produce pure oxygen and pure nitrogen. A major drawback of such cryogenic processes is that they require a great deal of energy and consequently are very expensive.
An alternate method that has been investigated for producing oxygen-enriched air involves selective permeation through polymeric membranes. Membranes are attractive for gas separations because of their low energy requirements and inherent selectivity. (Oxygen-to-nitrogen selectivity is defined as the ratio of oxygen permeability to nitrogen permeability.) However, because oxygen and nitrogen are such similar molecules, selectivities for oxygen over nitrogen are low with all polymeric membranes, usually between 1.5 and 4. See, for example, Hwang et al., Separation Science 9 (1974) 461. In addition, the most selective membranes have the lowest oxygen permeabilities. The most promising polymeric membrane has been silicone rubber, which has a selectivity of about 2 and an oxygen permeability of about 6.times.10.sup.-8 cm.sup.3 -cm/cm.sup.2 -sec-cmHg. This is the highest oxygen permeability of any polymeric membrane, but the maximum oxygen content of the gas produced from air is only about 35% regardless of the operating conditions. For this reason, polymeric membranes for the separation of oxygen and nitrogen have never been commercially successful.
A number of successful facilitated-transport methods are known for separating specific gases from gaseous mixtures by use of a complexing agent in a liquid membrane. See, for example, U.S. Pat. Nos. 3,844,735, 3,864,418, 3,865,890 and 4,239,506, all of which are directed to facilitated transport methods of separating ethylene from mixtures of methane and ethane. See also U.S. Pat. Nos. 3,396,510, 3,503,186 and 3,823,529, directed to similar methods for the separation of carbon dioxide, sulfur dioxide and carbon monoxide. Although the '510 patent to Ward et al. discloses the possibility of facilitated transport of oxygen, the proposed system is strictly an aqueous-based one, utilizing water-soluble complexing agents, and it was found to be commercially unfeasible.
It was observed by Tsumaki over forty years ago in Bull. Chem. Soc. Japan 13 (1938) 252 that synthetic chelate-type compounds reversibly bind oxygen. However, attempts to formulate a commercially feasible process for the production of oxygen- and nitrogen-enriched air using a membrane process have been unsuccessful to date.
The first demonstration of facilitated transport of oxygen across a membrane, using hemoglobin as the oxygen carrier, was reported by Scholander in Science 131 (1960) 585. The method reported was completely impractical, however, since hemoglobin is a protein which is easily denatured and not stable outside of the human body for longer than a few minutes. Moreover, hemoglobin is a very large molecule with consequent low diffusivity, which necessarily results in very low rates of transport of oxygen across the membrane.
In their pioneering study relating to facilitated transport of oxygen across a membrane, Bassett and Schultz reported selective transport of oxygen with the use of cobaltodihistidine as a complexing agent in an aqueous system in Biochim. Biophys. Acta 211 (1970) 194. However, the oxygen-to-nitrogen selectivity was only about 4, which did not represent an improvement over even polymeric membranes, and the liquid-membrane carrier system rapidly degraded, requiring the preparation of a fresh membrane for each separation. In addition, these membranes exhibited low oxygen permeabilities--less than that of silicone rubber membranes. Thus, this method also was impractical for the separation of oxygen and nitrogen from air.
It is therefore a principal object of this invention to provide a commercially feasible non-cryogenic process and apparatus for the separation of oxygen from atmospheric air or other oxygen-containing gaseous streams.
It is another principal object of this invention to provide a commercially feasible non-cryogenic process and apparatus for the separation of oxygen and nitrogen from atmospheric air.
It is still another object of this invention to provide a novel membrane useful for the separation of oxygen and nitrogen from atmospheric air.
These and other objects are accomplished by the method and apparatus and novel membrane of the present invention, which are summarized and particularly described below.